1. Field of the Invention
The present invention relates to circuitry for controlling the feedback path in a signal conditioning system.
2. Description of the Prior Art
There are many applications where a transducer generates a signal pulse which is utilized to drive a display or recording instrument. The signal pulse is usually superimposed on a d.c. level but the d.c. level is not constant since it is subject to electrical noise and signal drift. If the information to be displayed or recorded is proportional to the magnitude of the signal pulse, it is desirable to eliminate the d.c. level. A signal conditioning system is connected between the transducer and the instrument and has a feedback circuit for subtracting the d.c. level from the signal from the transducer. When the signal pulse is generated, the feedback path is interrupted to allow the signal pulse to pass through to the instrument.
One circuit which can be utilized to control the feedback path of a signal conditioner is disclosed in U.S. Pat. No. 3,772,604, issued on Nov. 13, 1973 to Walter R. Hogg and Wallace H. Coulter and entitled "Non-Rectifying Clamps." This circuit functions to maintain the output of a signal conditioner at a desired d.c. voltage level in the absence of signal pulses having magnitudes which exceed a predetermined reference voltage. The feedback circuit includes an operational amplifier connected between the signal conditioner output and input for generating a negative feedback signal.
A comparator compares the magnitude of the signal conditioner output signal with the magnitude of a reference voltage. When the output signal magnitude is below the reference voltage magnitude, the comparator will turn on an electronic switch to connect the signal conditioner output to the amplifier input to generate the feedback signal. When the output signal magnitude exceeds the reference voltage magnitude, the comparator will turn off the switch to interrupt the feedback path. A capacitor connected across the amplifier stores a charge for maintaining the d.c. feedback signal while the path is interrupted.
There are several problems which can arise when utilizing the previously described signal conditioning system. The output of the signal conditioner is coupled to the input of the comparator through a RC differentiator network. This circuit provides a timing out function to reconnect the feedback circuit to prevent the signal conditioning system from locking up with the feedback path interrupted. However, the operation of the circuitry is dependent upon the types of signals applied to the RC network. For example, if a noise signal has partially or fully charged the coupling capacitor of the RC network, a relatively low magnitude signal pulse would not be coupled to the comparator to turn off the electronic switch. Therefore, the feedback path would not be interrupted and the pulse signal would be eliminated from the output of the signal conditioner. Furthermore, if the signal pulse has a relatively low magnitude and a relatively long duration, the coupling capacitor would rapidly become charged and the comparator would turn on the electronic switch before the signal pulse terminated. Therefore, it can be seen that the interruption of the feedback path is dependent upon the duration of the signal pulse, which is desirable, and upon the magnitude and rise time of the signal pulse, which is not desirable as much of the signal pulse may be lost.